This invention relates to reactive systems for the production of cavity filling polyurethane foams for NVH (noise vibration and harshness) application areas in which the system exhibits reduced isocyanate emissions. These foams comprises a polyisocyanate comprising a polymethylene poly(phenylisocyanate).
Polyurethane cavity filling foams, particularly those having low densities, are suitable replacements for inserted baffles to inhibit noise transmission through automotive body structures as they offer advantages in terms of cost and performance. There is, however, a perceived exposure concern relative to isocyanate (MDI) emissions from the foam reactions for workers in the automotive assembly area where there foams are applied. MDI emissions are typically measured by the “5 gallon can test”. This test is documented in the paper titled “New Low MDI. Polyurethane Foam System for Acoustical Barrier Applications in the Automotive Industry” by Brad A. Pearson. This paper was presented at the May 1999 SAE International Meeting—Proceedings of the 1999 Noise and Vibration Conference.
Most earlier attempts to reduce isocyanate emissions in MDI based polymers have aimed to reduce the monomer content by forming a NCO-terminated prepolymer of MDI with an isocyanate-reactive component.
U.S. Pat. Nos. 6,423,755 and 6,541,534 disclose a polyurethane foam and a reactive system for forming a polyurethane foam. The system comprises (1) an isocyanate-reactive component including at least one polyol, a blowing agent and at least one thixotropic agent, and (2) a polyisocyanate component including at least one polyisocyanate compound (preferably an isocyanate prepolymer or quasi-prepolymer of polymeric MDI) and at least one thixotropic agent. At least one of the isocyanate-reactive component and the polyisocyanate component additionally contains hollow microspheres, and both have a Brookfield viscosity of at least about 50,000 cps. It is disclosed in these patents that the prepolymer approach to reduce the level of free MDI can substantially reduce or eliminate the need for engineering controls such as downdraft ventilation.
U.S. Pat. No. 6,803,390 discloses a method of making rigid polyurethane foams from a reactive system comprising (a) a polyisocyanate containing a prepolymer which is the reaction product of an excess of an isocyanate with at least one polyol and at least one hydroxyl-functional acrylate or methacrylate, and (b) a polyol component containing an effective amount of a blowing agent and isocyanate-reactive materials that have an average functionality of at least about 2.3 and include at least one polyol. These systems are also characterized by (c) a volume ratio of isocyanate to polyol of no greater than 10:1, and (d) a ratio of NCO to NCO-reactive groups of from about 0.8:1 to 1.5:1. It also requires that a catalyst be present in at least one of the polyisocyanate component or the polyol component. The presence of primary or secondary amine groups in the catalysts allows them to react into the resultant polymer structure and thereby decreases the level of volative components.
Rigid hybrid polyurethane foams are described in U.S. Pat. No. 6,699,916. These foams require volume ratios of less than 4:1 and are prepared from an NCO-terminated prepolymer in which the prepolymer is the reaction product of an isocyanate component, a polyol component and a hydroxyl-functional acrylate or methacrylate. This prepolymer preferably contains less than 25% of monomeric diisocyanates which substantially reduces the risks of polyisocyanate inhalation exposure.
Polyisocyanate prepolymers are also disclosed in U.S. Pat. Nos. 5,817,860 and 5,968,995. These prepolymers comprise the reaction product of (a) a polyisocyanate having a functionality of at least 2, and contain at least about 20% by wt. of a diisocyanate monomer, (b) a monohydric alcohol and (c) a polyol having an average hydroxyl functionality of at least about 1.8 to at most about 3.2. The prepolymer is characterized by (i) a sufficient quantity of isocyanate groups to react with water in the absence of a supplemental blowing agent to make a foam, (ii) at most about 10% by wt. of diisocyanate monomer, and (iii) a sufficient amount of polyisocyanate capped by monohydric alcohol to prevent gelling of the prepolymer.
Advantages of the present invention include the ability to control and/or reduce the emissions of free MDI in cavity filling foams. The present invention does not require prepolymers of polyisocyanates or other conventional approaches to this problem, but achieves significant reductions in the emission of free MDI.